Location-based services and emergency call positioning drive the development of positioning in wireless networks. Positioning support in 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) was introduced in Release 9. The introduced positioning support enables operators to retrieve position information for location-based services and to meet regulatory emergency call positioning requirements.
Positioning in LTE is supported by, e.g., the architecture in FIG. 1, where direct interactions between a wireless device (e.g., a UE) and a location server, e.g., an Evolved-Serving Mobile Location Center (E-SMLC), are via the LTE Positioning Protocol (LPP). Moreover, there are also interactions between the location server and the eNodeB via the LPPa protocol, which to some extent are supported by interactions between the eNodeB and the device via the Radio Resource Control (RRC) protocol.
The following exemplary positioning techniques are considered in LTE:                Enhanced Cell ID: Essential cell ID information to associate the wireless device to the serving area of a serving cell, and then additional information to determine a finer granularity position.        Assisted Global Navigation Satellite System (GNSS): GNSS information retrieved by the wireless device, supported by assistance information provided to the wireless device from E-SMLC.        Observed Time Difference of Arrival (OTDOA). Wireless device estimation of the time difference of reference signals from different networks node such as base stations and sending of such estimations to the E-SMLC for multi-lateration.        Uplink Time Difference Of Arrival (UTDOA), wherein a wireless device is requested to transmit a specific waveform for detection by multiple location measurement units, e.g., a base station, at known positions, and forwarding of these measurements to E-SMLC for multi-lateration.        
Global positioning system (GPS)-enabled terminals can meet the requirement for positioning, but they cannot provide the required availability when the wireless is being blocked to the satellite signals in urban and indoor environments. Therefore other techniques are needed in such environments. OTDOA has been introduced in 3GPP release 9 as a downlink (DL) positioning method. As illustrated in FIG. 2, OTDOA in LTE is based on the wireless device measuring the time of arrival (TOA) of signals received from network nodes such as eNBs. The wireless device measures the relative difference of TOA between a reference cell and another specific cell, defined as reference signal time difference (RSTD) measurement. Every such RSTD measurement determines a hyperbola, and the intersecting point of these hyperbolas can be considered as the position of the wireless device. Here, the wireless device selects the reference cell, where the RSTD measurement can be performed on an intra-frequency cell, wherein reference cell/neighbor cell are on the same carrier frequency as the serving cell, or inter-frequency cell, wherein at least one of reference cell/neighbor cell is on a different carrier frequency from that of the serving cell.
The OTDOA-based positioning accuracy depends directly on the accuracy of the RSTD measurements. While having device RSTD measurements with respect to more cells is generally viewed more beneficial because the amount of the information about the wireless device location increases, the accuracy of the result still relies on the precision of the RSTD estimations for all the RSTD measurements. The RSTD accuracy is largely dependent on the type of cells that the wireless device measures. Because the type of cell is not known at the wireless device, results for e.g. mixed-cell scenarios may experience performance degradation, e.g., when small and large cells are present in the area or the wireless device can be in indoor and outdoor environments, leading to a poor accuracy when determining a location of the wireless device.